linux-kernel - Re: [PATCH] virtio_mem: break device on remove

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Date:   Mon, 17 Jan 2022 02:55:42 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org, David Hildenbrand <david@...hat.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH] virtio_mem: break device on remove

On Mon, Jan 17, 2022 at 02:40:11PM +0800, Jason Wang wrote:
> 
> 在 2022/1/15 上午5:43, Michael S. Tsirkin 写道:
> > A common pattern for device reset is currently:
> > vdev->config->reset(vdev);
> > .. cleanup ..
> > 
> > reset prevents new interrupts from arriving and waits for interrupt
> > handlers to finish.
> > 
> > However if - as is common - the handler queues a work request which is
> > flushed during the cleanup stage, we have code adding buffers / trying
> > to get buffers while device is reset. Not good.
> > 
> > This was reproduced by running
> > 	modprobe virtio_console
> > 	modprobe -r virtio_console
> > in a loop, and this reasoning seems to apply to virtio mem though
> > I could not reproduce it there.
> > 
> > Fix this up by calling virtio_break_device + flush before reset.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> >   drivers/virtio/virtio_mem.c | 2 ++
> >   1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
> > index 38becd8d578c..33b8a118a3ae 100644
> > --- a/drivers/virtio/virtio_mem.c
> > +++ b/drivers/virtio/virtio_mem.c
> > @@ -2888,6 +2888,8 @@ static void virtio_mem_remove(struct virtio_device *vdev)
> >   		virtio_mem_deinit_hotplug(vm);
> >   	/* reset the device and cleanup the queues */
> > +	virtio_break_device(vdev);
> > +	flush_work(&vm->wq);
> 
> 
> We set vm->removing to true and call cancel_work_sync() in
> virtio_mem_deinit_hotplug(). Isn't is sufficient?
> 
> Thanks


Hmm I think you are right. David, I will drop this for now.
Up to you to consider whether some central capability will be
helpful as a replacement for the virtio-mem specific "removing" flag.

> 
> >   	virtio_reset_device(vdev);
> >   	vdev->config->del_vqs(vdev);